Block signal system



Feb, 11 1936. w QARKENBURGH 2,030,675

BLOCK SIGNALSYSTEM Original Filed Dec. 5, 1929 2 Sheets-Sheet l 29% (Eg zvLN A Q LR U 25 [W E Z0W 7 I, V 20"W Q Q If wvwv ilyvvwww 0 i v 5 l l A Inventor: v Weber- H. Awkenburgh, W QM His Attorney Feb. 11, 1936. w, ARK NB GH 2,030,675

BLOCK SIGNAL SYSTEM Original Filed Dec. 5, 1929 2 Sheets-Sheet 5 Fig. 5. 24' a? 1 Inv ehtor: Weber H. Ar'kenburgh,

Patented Feb. 11, 1936 UNITED STATES PATENT F'IifiE.

BLOCK SIGNAL! SYSTEM Weber H. Arkenburgh,

Schenectady, N. Y., .as.-

signer to General Electric Company, a. corporation. of New York,

14 Claims.

My invention relates to railway blocksignal systems, and more particularly to railwayblock signal systems the operation of which is entirely electrical.

It has for one of its objects to provide a railway block signal system which is entirely electrical in its operation and which is operable selectively to control the block signal of a railway system as desired in signal practice and which includes a single line circuit extending between adjacent blocks.

A further purpose of the invention is to provide improved means for controlling the signals in each oftheblocks. Thus, in accordance with my invention a signal circuit is provided in each block including a plurality of branches each of of the different signals being connected in a different branch of said circuit, together with means, which may comprise electron discharge means of suitable type whereby the current in each branch is controlled as desired in accordance with track conditions.- The electron discharge means provided as indicated preferably comprise a pair of vapor electric devices, or electron discharge devices of other suitable type, one of which is controlled in accordance with track conditions in the block in which it is located and the other of which is controlled in accordance with track conditions in an adjacent block. These discharge devices are so connected and arranged with reference to the branches of said circuit that the energization of the different signals, or other electroresponsive devices provided in the different branches are controlled bysaid track conditions.

In this specification I shall particularly mention the. operation of the signals in accordance. with track conditions and shall describe a particular means of controlling the different discharge devices in accordance with track conditions. It will of course be understood, however, that I do not.

, method of operation, together with further objects is divided into blocks A,.B,.' etc. the rails of'adjacent blocks beingzinsulated' from each. other'in accordance with common practice. as-indicated at the points Land 3. It will beassumed for :the. purposes of the following description that traffic,

upon the trackal passes in the direction of the arrow indicated'in block A- At land-"5:1 have indicated the signal equipment which is: associated with each of theblocks B and C respectively. These equipments are identical in character and it will be understood that a-simil'ar equipment is associated with each block of the railway system. For the purpose of simplification of the drawings only two equipments'are represented; The usual signals corresponding to stop; caution and clear conditions of the track areindicated at R; Y and G, R, Y' and G, etc. in-the equipment associated with eachof the block's. Also included in each of these equipments is a pair of electron discharge devices indicated in theequipment 5 at 8 and 9 and inthe equipment 4' at 8'-and 9. Also employed in each ofthe-signal equipments is a dischargedevice l0; I0 etc. the function of which is, as will later be explained, somewhat different from that of the discharge devices 8, 9 and 8', 9", and which may, or may-"not, be of the same type of device. As-willbe-made clear in the description and as illustrated in Fig. 2 the discharge device H1; H1" may be omitted if "de-- sired.

The discharge devices employed may-be of the heated cathode vacuum type such asare'employed commonly in signal systems, althoughl prefer to employ vapor electric devices or devices which the enclosing envelope is provided with'a trace-of mercury vapor. These dischargedevices have the characteristicthatas the gridis gradually made positive. withtrespect to the cathode they suddenlyebecome conducting; andwhen made negative they becomenon-conducting. at substan v tially the voltage at which they became conducting. T'hatyis atas fixed'value of grid; voltage which is substantially unique, the impedance of the device changes-from a substantially. infinite value to array low value and-visaversa, and the 9 and ID are energized through separate sec ondary windings ll, l2, and I3, ll, I2 and I3 respectively, on transformers [4, J4, the primaries of whichare connected across a pair of power supply conductors 15. The right-hand winding of each of the transformers l4, I4 is connected 'to supply electromotive force directly to the'track rails of the adjacent block in the direction opposite to the direction of trafiic. 7 At the opposite end of each block this electromotivc force is impressed upon the primary winding of a transformer 20, 20 through a transformer 23, 23 and amplifier I0, Ill". The secondary windings l9, I9 of transformer 20, 20' are connected to'suppl'y this potential to the grid of the assoc'iated'discharge device 9, 9 respectively. This grid circuit in the equipment 4 extends from the cathode of device 9 to the midpoint of winding.

I2 and thence through the winding E9 to the grid of device 9. Thispotential is such that when supplied to the grid of device 9 this device is of very low impedance, whereas when this potential is interrupted as by a vehicle entering the corresponding block and thereby short circuiting the rails through the Wheels and axles'of the vehicle, the discharge device is of very high impedance.

The. secondary winding 25' of the transformer 20' and, similarly, each of thecorrespondingwindings in the different signal equipments, is connectedto supply potential to the grid of device 8 in the adjacent blockin the direction opposite to the direction of traffic through a circuit ex tending from one side of Winding 25,through line conductor 24, grid and cathode of device 8, and common conductor I6 to the opposite side of the winding 25,. Thus the devices 8, B in the different blocks are controlled similarly with devices 9, andt, but'in response to the condition of the adjacent block in the direction of traffic.

. It will now be apparentthat the gridcircuits of these devices are inherently safe, that is, any open circuit such as might be caused by a broken rail in the corresponding block or a short circuit of the grid circuit at any point between the supply conductors and'the different grids has the same effect upon the signal system as the presence of-avehicle in the corresponding block 1. c. it causes, the supply-of .electromotive force upon the respective grids to be interrupted.

The discharge devices 8 and 9 are connected in series through a circuit which extends from the secondary winding 11 on transformer I4, anode and cathode of discharge device .9, anode and cathode of discharge device 8, signal lamp G and resistance l8 back to the opposite side of winding l'l. If we assume that there is no vehicle inthe block Bior; in block C, sufiicient current will flow circuit extending from resistance l8 through lamp Y, resistance 21 to the cathode of device 9, the

circuit being completed through device 9, and

winding l1. Similarly current flows througha 7 The system may now best be understood by reference to its operation. When a vehicle enters the block B, for example, the primary winding of transformer 23 associated with that block becomes short circuited thereby removing alternating current from the grid of amplifier l0 and hence from the grid of the device 9 and also from the line conductor 24 extending to equipment associated with block A (not shown). 'Thus, the above traced circuit through the signal lamp G is now interrupted by the high impedance of the discharge device 9, andthe current in resistance I8 is considerably reduced. The device 9 also interrupts the circuit of lamp Y preventing that lamp from lighting. Itwill be noticed that signal lamp R is connected directly across the winding I! through resistances l8 and 6. The reduction of potential on the resistance l8 causes a greater proportion of the potential of winding, I! to be impressed across the signal R in the equipment 4 and an increase of current in this branch of the circuit. This lamp isthus lighted indicating that the block B is occupied. Z

Let us assume that the vehicle moves from the block B into the block 0. It will of course be apparent that the clear or safe signal G asso ciated with the block C is extinguished and'the lamp R lighted. Deenergization of the transformer 20' of the equipment removed alternating current potential 'from the grid of the discharge device 8 in the equipment 4, and thereby causes this discharge device to become of high impedance. Alternating current is again supplied to the grid of the discharge device 9 causing this device to be of low impedance. Signal lamp R in the equipment 4 is now shunted 'by the above mentioned branch circuit extending through the lamp Y in said equipment, resistance 2 l, cathode and anode of discharge device 9 'to one terminal of the secondary winding. ll. The current inthis circuit causes the lamp Y to be lighted and sumcient drop in potential in resistance l8 to cause the lamp R to be extinguished. The lamp G at this time is extinguished since the discharge device 8 which is connected in series therewith is now of high impedance. nals indicates to a vehicle operator proceeding from blockA that block B is unoccupied and that block C is occupied. When the vehicle moves from block C into: the next adjacent block to the left it will be apparent that the lamp G in the equipment 4 will again be lighted and the lamp Y extinguished. The lamp Y in the equipment 5 will become lighted and remain lighted untilthe vehicle leaves said next adjacent block. It will of course 'beapparent that when both blocks B and C are occupied, a danger or stop signal will be given by both the equipments 4 and 5.

Referring now to Fig. 2 a somewhat different arrangement of the circuits is employed, although substantially the, same operation occurs. In the system shown in Fig.2 the amplifiers. l0, In" of Fig.1 are omitted, potential being supplied from the track directly through the transformdis 2!), to the grid of the discharge devices 9, Q respectively in the equipments4 and 5. Alternating current potential is similarly supplied to the grid of the discharge device 8 in the equip- This condition of sigment, 4 from a secon ary winding 25' on the transformer 28' in the equipment 5 through line conductor 24. and the common conductor l6. In this figure discharge devices of the heater cathode type are indicated, although of course other types of discharge devices may be used as well. The heating circuits of the devices 8 and 9, 8 and 9 are connected to the secondary windings H and 12, H and I2 on the transformers l4, l4 respectively. The anode circuit of the discharge device in equipment 4 extends from one side of the secondary winding I! of transformer l4 through resistance 26, cathode and anode of discharge device 9, cathode and anode of discharge device 8, and the clear signal G to the opposite side of the winding I1. Thus the signal G in each of the blocks is normally lighted. Connected in shunt with discharge device 8 is a branch circuit including resistance 21, and signal Y. The circuit of this branch is completed through device 9, resistance 26, winding H, and the signal G; Similarly connected in shunt with discharge device 9 is a branch circuit comprising resistance 29 and primary winding of transformer 28, the circuit of this branch is completed through resistance 21, winding l1, and the signal G. The resistances 21 and 29 in these branch circuits and also the resistance 26 are so adjusted that the signals Y and R are not sufficiently energized to cause them to light when alternating current is supplied to the grids of the discharge devices 8 and 3 and these devices are of low impedance. The signal G is caused to light only when both of discharge devices 8 and 9 are of low impedance and maximum current flows through the signal device.

Let us assume now that a vehicle enters the block B from the block A. It will be seen that alternating current is then removed from the grid of the discharge device 9, and likewise from the line conductor 24 leading to the equipment (not shown) in the block, A. The device 9 becomes of high impedance such that the greenlight is extinguished, and the potential. drop in, resistance 26 reduced. The primary winding on transformer 28, which was previously bridged by the anode-cathode circuit of the discharge device 9 is now sufiicie-ntly energized in series with device 8 and lamp Y connected in parallel, with the signal G to cause the lamp R to light thereby causing a stop signal to be given. The current in the lamp Y will not be sufficient to cause it to light, since the discharge device 8' in, shunt therewith is still supplied with alternating currents and is of low impedance. The condition of signals in the block C remain unchanged.

Now let us assume that the vehicle passes from block B into block C. The sameoperations of. course occur in equipment 5, viz: the lamp; G. is extinguished and the lamp R lighted at that block and at the same time alternating current is,

removed from the conductor 24' extending between blocks C and B thereby causing the discharge device 8 to become of high impedance. When this occurs the low impedance shunt to signal Y comprising the anode-cathode circuit of the discharge device 8, becomes one of high impedance thereby reducing the current through the lamp R and, accordingly, maintaining; the

reduced drop in potential in the resistance 29. An increased current will now flow through lamp Y and discharge device 9 thereby causing,

lamp Y to be sufficiently. energized;to light t-hereby toprovide a caution signal to vehicles a-p,

preaching from block A, When; the vehicle moves into the block to the left of block, 0 normal conditions are,- restored in the equipment 4 and thecaution signalis given in equipment 5.

In.- .the equipment indicated in Fig. 1 resistances 6, 2|. and 6', 21- are shown connected in series with signal lamps R, Y and R, Y and other resistances 4|- and 42 in shunt to lamps Y and G. These resistances are of value such that the signal lamps mai all be of equal impedance. The resistance 42 is inserted to reduce the impedance of the circuit branch including lamp G so that when it is lighted the yellow lamp is extinguished, and similarly the resistance 4| is inserted to reduce the impedance of the branch including the lamp Y so that when it is lighted the red lamp R is extinguished. It will immediately be seen therefore that if when the lamp G is lighted, it burns out, the ye?low lamp Y will be lighted; and similarly if when the yellow lamp Y is lighted, it burns out, the red lamp becomes lighted. When the red lamp R burns out, no signal is given. This is what is known as progressive failure of the signals. That is, upon failure of any signal an indication is given which is more restrictive to vehicles than would be given if the apparatus functioned properly. While if the red lamp R burns out no signal is produced, it, is to be noted that the absence of a signal is the most restrictive indication given, being even. more restrictive than the redv signal.

Similarly in the equipments shown in- Fig. 2 the caution and clear signals Y and G have resistances connected in parallel therewith and the caution and clear signals have resistances inseries therewith for the same reason to permit the use of lamps of equal impedance. Preferably the transformers 20 of the systems of Figs. 1 and 2, have perrnalloy cores such that the core becomes saturated upon a comparatively small portion. of the voltage normally supplied thereto. It will be apparent that when transformers of this type are employed the system is rendered comparatively insensitive to variations in the voltage normally appliedto the track.

In these figures the clear signal is normally indicated in each of the blocks, this. indication being changed to stop when a vehicle enters a particular block, and to caution when the vehicle leaves the block and enters the next block in advance of the vehicle provided no other vehicle enters; the former block. These results are efiected, it will be noted, by means employing a.

single line circuit extending between adjacent blocks.

In Fig. 3 I have shown means whereby the various changes in signals may be effected by means employing a single electron discharge device in each block and a single line circuit extending between adjacent blocks. In this figure the signals are all normally extinguished and are changed upon the approach of a vehicle in accordance with track conditions in theblock in. advance of the: vehicle.

Referring to Fig. 3 I have shown discharge device 38 in the equipment 4' and device 38 in equipment 5. The anode and cathode circuits of these discharge devices are supplied from the secondary winding of, the transformers l4, I4 as was the case in the previous figures and, the circuits of the different signal lamps are energized' in parallel circuits through transformers 30, 30 from the secondary winding I], ll of transformer l4, M. The circuit of the equipment14 and, similarly, of eachof'the-other equip! ments. includin the se on y nd ng: 1; and.

'u'rated core type.

the primary winding of the transformer in cludes also a resistance 3|, the opposite ends of which are connected to the two rails of the adjacent block in the direction opposite to the direction of traffic. The value of this resistance is such that normally insufiicient electromotive force is supplied, to the primary winding of the transformer 33 to cause lamps R, Y and G; R, Y and G to be lighted, although when the block to the right of the respective signal system is well-known construction, but are of a type such that when direct current flows in one of the windings thereof, which I will call the direct current V winding, of sufiicient value to saturate the core;

the other winding, which I will call the alternating current winding, is of very low impedance to alternating current, whereas when there is no direct current flowing in said winding and the core is unsaturated, the alternating current winding is of very high impedance. The direct current windings of each of these reactors are normally energized as will presently appear, and. accordingly, their alternating current windings are of very low impedance to alternating current. It will be seen from the drawings that the alternating current winding of the reactor 32 is connected in parallel with the lamp R. The impedance of this winding when direct current is flowing in the primary circuit is such that the lamp R is prevented from lighting that is, it is virtually short circuited. Similarly it will be seen that the secondary winding of the reactor 33 is connected in parallel with the lamp Y and, accordingly, this lamp is, in effect, short circuited. The alternating current winding of reactor 34 is connected in series with the lamp Y and a limiting reactor 31 but, since the lamp Y is short circuited, it does not become energized. The reactor 3'5 prevents an excessive current from flowing in the circuit. The alternating current windings of reactors 35 and 3Bare both connected in series with the lamp G, and are normally of low impedance, but since, as may be assumed, there is no vehicle in the block A, the resistance 3| is not short circuited and, accordingly, the lamp G is not lighted.

Let us now asume that a vehicle enters the block A and that blocks B and C are unoccupied. Resistance 3| is short circuited by the vehicle, and the lamp G becomes lighted. Lamps R and Y; will not be lighted for reasons already explained. I

The anode circuit of the discharge device 38, which is identical with the anode circuit of each of the other discharge devices, can best be understood by tracing the circuit of discharge device 38' since this circuit is completely illustrated in the drawings. This circuit extends from the cathode of discharge device 33 through the direct current windings of the reactors 34, 355 and 32' in the equipment 5,.line conductor 24f extending between blocks C and B, direct current windings of reactors 33 and 35 in the equipment 4, thence over the common conductor l6 and middle secondary winding of transformer l4 to the anode of discharge device 38'. Thus the anode circuit of the discharge device 38 extends from the cathode thereof through the direct current windings of reactors 34, 36 and 32, and thence to the equipment (not shown) associated with the block A after which it returns through the common conductor l8 and the middle winding of the transformer l4 to the anode of the discharge device'38.

Now let us assume that the vehicle enters block B and leaves block A unoccupied. The resistance 3| becomes short circuited and, since the discharge device 38' is energized, the lamp G will become lightedthereby indicating a safe condition in the advance block C. Alternating potential is now removed from the grid of the discharge device 38 and this device becomes of high impedance, thereby interrupting current in the resistance 3| will be short circuited and sumcient energy will be supplied to the transformer 30 and thence through limiting reactor 39 to cause the red lamp R to be lighted, thereby informing the vehicle operator in block A that the advance block is occupied. Thus with a. vehicle in both block B and block A, the lamp G Will be lighted in the equipment 5' and the lamp R will be lighted in the equipment 4. a

Let us assume that the vehicle in block B now enters block C. The discharge device 38 now becomes of high impedance thereby deenergizing the direct current windings of reactors 34', 36 and 32' in the equipment 5 and reactors 33 and 35 in the equipment 4. equipment 5, however, since the resistance 3| is not short circuited, the block B being unoccupied. Potential is again supplied.- to the grid of discharge device 38, thereby causing the direct current windings of reactors 34, 36, and 32 in equipment 4 to be energized. Since resistance 3| is now short circuited, lamp Y will be lighted in equipment 4, and the lamp R will be extinguished since it is new short circuited by the secondary winding of reactor 32. The'lamp G will not be lighted since the direct current winding of reactor 35 is deenergized. v t

It will thus be seen that in this system the signals are normally deenergized, but that as i the block immediately in advance is occupied its signal indicates"-danger. 'The danger signal in the block occupied by a vehicle, is not given except when the block immediately to the rear of the vehicle is occupied. The caution signal in any block will be given only when both adjacent blocks are occupied and the block in which the signal is located is unoccupied.

If desired the arrangement shown in Fig. 3

may be modified as shown in Fig. 4'by eliminating transformer 30 andenergizing the signal No signal is lighted in circuits directly from the winding I! on transformer M. The resistance 3| and connections thereto leading from the track circuit are omitted. As thus modified it will" be seen that the clear signals will be normally lighted and will be extinguished by a vehicle occupying the corresponding block thereby causing the stop signal to be lighted, or by a vehicle entering the ad-. Vance block thereby causing the caution signal to be lighted.

While I have shown a particular manner of supplying the control electromotive force to the discharge device, it will of course be understood that this may be effected in any other suitable Way, it being necessary only that the condition of the discharge device be altered by the presence of the vehicle in the particular block with which the signal is associated.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications, both in the circuit arrangement and in the instrumentalities employed, may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination, in a railway block signal system of a plurality of discharge devices in each block ofsaid system, a plurality of signaldevices in each block, a source of electromotive force, a circuit including said discharge devices connected in series relation and said source of electromotive force, said circuit haying a plurality of branches, one of said signal devices being included in each of said branches, and means including said discharge devices for so controlling said branches that said signal devices indicate conditions in the different blocks.

2. The combination in a railway block signal system of a plurality of series connected discharge devices in each block, a line circuit extending between adjacent blocks, one of the discharge devices in each block being connected to respond to conditions in the respective block and the other discharge device being connected to respond to conditions in said line circuit, means in the adjacent block for controlling the condition of said line circuit in accordance with the conditions in said block, clear, caution, and danger signals in each block, and means for selectively operating any one of said signals in response to the impedance of said discharge devices.

3. In combination, a circuit including an electroresponsive device and a plurality of series connected electron discharge devices, a plurality of branch circuits, each of said branch circuits including an additional electroresponsive device and certain of said electron discharge devices, and means for controlling the impedance of said discharge devices thereby selectively to operate said electroresponsive devices.

4. In combination, a circuit including a plurality of series connected discharge devices and a source of electromotive force variable dependently upon the impedance of said circuit, a plurality of shunt connections to said circuit, an electroresponsive device in each of said shunt connections, said shunt connections being arranged for selective energization in response to the conductivity of said discharge devices and means for controlling said discharge devices trolling another of said discharge devices, said branch connections being so connected and ar-. ranged that said signal devices are selectively controlled in accordance with the condition of said discharge devices.

6. The combination, in a railway signal system of a plurality of signals in each block, a circuit including a source of electromotive force variable dependently uponthe impedance of the circuit, a plurality of series connected discharge devices arranged to control said impedance, shunt circuits about said discharge devices, one of said signals being included in each of said shunt circuits, and means responsive to track conditions in each of a plurality of blocks to control said discharge devices thereby to cause operation ofsaid signals in response to variations in said source.

'7. The combination, in a railway block signal system of a source of electromotive force in each block, a plurality of electron discharge devices connected in series across each of said sources, means for controlling the conductivity of one of said, discharge devices in each block in response to track conditions in the respective block and means for controlling another of said discharge devices in response to track conditions in an adjacent block, and a shunt circuit around each of said discharge devices, each of said shunt circuits including a signal device, and impedances so comiected in circuit with said discharge devices and signals that said signals are selectively operated in accordance with the conductivity of the diiferent discharge devices.

8. The combination, in a railway block signal system of a circuit in each block comprising a source of electromotive force, a pair of normally conducting discharge devices and a normally actuated signal device all connected in series, means to render one of said discharge devices of each pair nonconductive in response to track conditions in the respective block and to render the other of said devices nonconductive in accordance with track conditions in an adjacent block, and additional signal devices connected in shunt paths about said discharge devices, the regulation of said source and the impedance of said shunt paths being so proportioned that when either of said discharge devices is nonconductive the energization of said normally energized signal device is substantially reduced and one or another of said additional signal devices is actuated dependent upon which of the discharge devices is nonconductive.

9. The combination, in a railway block signal system of a circuit in each block comprising a source of electromotive force, a signal device, and a pair of electron discharge devices all connected in series, and a plurality of shunt paths to said circuit, each of said paths including an additional signal device, and impedances so distributed in said circuit and shunt paths thereto that said signal devices are selectively actuated in accordance with the conductivity of said discharge devices and' means for controlling the conductivity of one of said discharge devices in each block in accordance with track conditions in the respective block and the conductivity of the other of said discharge devices in accordance with track 7 conditions in an adjacent block;

10. In a railway block signal system, the combination of clear, caution and danger signals in.

each block means all of the component parts of which are stationary with respect to each other for energizing the danger signal when the respective block is occupied and for selectively energizing the caution and clear signals dependent upon the track condition of an adjacent block, said meansincluding means responsive to failure of any of said signals to produce an indication which is more restrictive than the indication produced by the signal which failed all of the component parts 'of said last means remaining stationary with respect to each other during said response to said failure.

11. In a block signal system, the combination of a circuit in each block having a plurality of branches, clear, caution and danger signals connected respectively in different branches of said circuit, and means all of the component parts of which are stationary with respect to each other for so controlling said circuit branches that different of said signals are energized in accordance with different track conditions in the respective block and .an adjacent block, and means operable when any one of said circuit branches is opened thereby disabling the respective signal to produce a more restrictive indication than is produced by the signal in the opened branch, all of the component parts of said last means remaining stationary with respect to each other during operation thereof to initiate said more restrictive indication.

12. In a railway block signalsystem, comprising a plurality of blocks, the combination of clear, caution and danger signal lamps in each block, means all of the component parts of which are stationary with respect to each other for selectively controlling said signallamps-to cause the V difierent lamps to be lighted in response to diiferent corresponding conditions of the track in the respective block and an adjacent block, and means responsive to failure of any one of said lamps to give a more restrictive indication than is given by the lamp which failed, said last means operating while all of said component parts rethe caution lamp islighted, when the caution lamp burns out the danger lamp is lighted, and when the danger lamp burns out all of said lamps are deenergized, said last means operating while all of said component parts remain stationary.

14. The combination, in a block signal system, of a circuit network in each block including clear, caution and danger signal devices in different portions thereof, and a source of electromotive force for energizing said devices, means entirely electrical to-control the electromotive' force supplied to said diiferent portions to energize the danger signal device in response to track conditions in the respective block, and to selectively energize said clear and caution signal devices in accordance with track conditions in an adjacent block,

and means comprising impedances in the different portions of said network so arranged that in response to failure of the clear signal device the caution signal device becomes energized from said source, and in response to failure of the caution signal device the danger signal .device is energized from said source, said last means operating while all of its component parts remain relatively.

stationary.

. WEBER H. ARKENBURGH.

iii). 

